The Microsoft Preferred Architecture for Exchange Server

In addition to providing the minimal amount of infrastructure for high availability,
you may want to consider the Microsoft preferred architecture for Exchange Server
2013 (Exchange PA).
Although the Exchange PA calls for running Exchange on dedicated physical servers,
it also includes many design aspects that can be beneficial in any environment.
The Exchange PA includes the following design requirements (source: Microsoft Exchange team blog):

Includes both high availability within the data center, and site resilience between
data centers

Supports multiple copies of each database, allowing for quick activation

Reduces the cost of the messaging infrastructure

Increases availability by optimizing around failure domains and reducing complexity

You can think of AWS Availability Zones as separate physical data centers.
Following the Exchange PA to architect your Exchange Server deployment on AWS provides
the added ability to have high availability within a single AWS Availability Zone,
as well as across zones in your AWS region.

Figure 2: Exchange Server 2013 Architecture on AWS based on the Exchange PA

Following the Exchange PA, each database has four copies, with two copies in each
Availability Zone.
This means that the Exchange PA requires four servers at a minimum, even for a small
deployment of 250 users.
Out of these four copies, three servers are configured as highly available (HA) copies,
and the fourth server is a lagged database copy configured with the
ReplayLagTime parameter set to up to 14 days.

Keep in mind that when you design a solution based on the Exchange PA, you end up
with an architecture that provides the highest amount of availability, but that also
includes a significant amount of infrastructure.
As you will see later in this guide, we provide some sample design scenarios so you
can get an idea of how much infrastructure is involved.
You can use these scenarios to customize your own design.

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